A complex sweetener electrochemical modified electrode, a preparation method and application thereof

Abstract

The application provides a complex sweetener electrochemical modification electrode and a preparation method and application thereof. The NO2-MIL-53(Cu-Al) bimetallic organic framework material is prepared by a simple one-step hydrothermal method, and is used as an electrochemical active material to prepare the NO2-MIL-53(Cu-Al) modified modification electrode on a glassy carbon electrode by a drop coating method. The coupling effect between the two metal ions is used to improve the conductivity and catalytic ability of the material, and an electrochemical sensor capable of rapidly and sensitively detecting the complex sweetener and sucralose is constructed. In addition, in order to verify the practical application ability of the modified electrode, quantitative analysis is carried out on the actual sample containing the complex sweetener, and it is proved that the sensor has good anti-interference ability and recovery rate, which is consistent with the determination result of high performance liquid chromatography. Meanwhile, the method is low in cost, green and environment-friendly, and simple in process, and provides a new method for rapidly and sensitively detecting the complex sweetener in the actual sample.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of electrochemical sensing technology, and belongs to the field of detection technology of food, pharmaceuticals, feed and the like. BACKGROUND

[0002] New hesperidin dihydrochalcone is a natural sweetener obtained from citrus flavanones, naringin and neohesperidin. It is characterized by high sweetness, stable properties and refreshing taste. Its sweetness is about 1000 times that of sucrose. Compared with sucrose, its sweetness comes slowly, disappears slowly, and has a long aftertaste, sometimes with a bitter aftertaste similar to glycyrrhizinate and menthol. Due to the characteristics of dihydrochalcone, such as high sweetness, extremely small dosage, and safety in toxicological analysis, it can be applied in food, beverages, candies and medicines. Although current research results show that NHDC has no significant harmful or side effects, according to the health standard for use of food additives of the European Union, its maximum allowable daily intake (ADI) is 5 mg / kg. Sucralose, commonly known as sucralose, is the only functional sweetener using sucrose as raw material, which can reach 600 times the sweetness of sucrose. It has high stability, pure sweetness, and sweet taste that appears quickly, sweet duration, aftertaste and other sweet characteristics similar to sucrose, without any bitter aftertaste. Sucralose can be used in many fields in China, such as beverages, pickles, compound seasonings, prepared wine, ice cream, cakes and the like. However, while meeting the needs of people, sucralose also has some problems in application that need to be solved urgently, such as the production of pollutants that are difficult to degrade, the impact on the human metabolic system and genetic damage, and other hazards. Therefore, the World Health Organization (WHO) stipulates that the maximum allowable daily intake (ADI) of SUC is 13 mg / kg. Based on the above discussion, people usually add NHDC and SUC simultaneously in practical applications to ensure that the sample has good taste and that each kind of sweetener added does not exceed its limit value.

[0003] Therefore, in order to ensure the health of people, it is urgent to establish a detection technology that can quickly determine the content of NHDC and SUC in actual samples. At present, the analysis methods for monitoring the addition amount of NHDC and SUC include high performance liquid chromatography, capillary electrophoresis and electrochemical method, etc. Among them, electrochemical method as a new type of analysis method has been favored by more and more people due to its rapid response, simple operation, time saving, low cost of instrument, high sensitivity and good selectivity, and therefore it has considerable application prospect in the safety detection of NHDC and SUC.

[0004] In the process of sensor construction, the working electrode is the key. Because of the poor electrochemical response of the bare electrode, researchers use various modified or assembled functional materials to modify the electrode to improve the charge transfer rate of the material, thereby enhancing the redox reaction of the target, enhancing the sensitivity of the electrochemical sensor, reducing the detection limit and expanding the linear range. MIL-53 series of MOF materials not only have a large specific surface area, but also have excellent stability in water and high temperature conditions. In addition, bimetallic MOFs with different active metal sites exhibit superior physical and chemical properties and synergistic effects compared to their monometallic counterparts. Moreover, when the appropriate metal nodes are selected, the structure and composition of the bimetallic MOF can exhibit high stability, good electrical conductivity and exposed active sites. Therefore, using bimetallic MOF materials as electroactive substances to modify bare electrodes to detect complex sweeteners in actual samples is a hot research direction. SUMMARY

[0005] The purpose of the present application is to provide a complex sweetener electrochemical modified electrode and its preparation method and application. By preparing Cu / Al bimetallic organic framework material; for detecting complex sweeteners (NHDC and SUC), providing a method for analyzing complex sweeteners (NHDC and SUC) in actual samples with high sensitivity, good selectivity, simplicity and speed. The present application uses a simple one-step hydrothermal method to prepare NO2-MIL-53(Cu-Al) material, and uses it as an electrochemically active material. A NO2-MIL-53(Cu-Al) modified electrode is prepared by a simple drop coating method. It is used as a working electrode, a saturated calomel electrode is used as a reference electrode, and a platinum wire electrode is used as a counter electrode to assemble a three-electrode system. The electrolyte is a pH=5.8 MBS buffer solution prepared by mixing 0.1 mol / L citric acid and 0.2 mol / L disodium hydrogen phosphate solution according to a certain volume, and is connected to an electrochemical workstation for electrochemical detection of complex sweeteners (NHDC and SUC).

[0006] A complex sweetener electrochemical modified electrode for sweeteners NHDC and SUC and products containing NHDC and SUC mixed, the electrochemical sensing electrode comprises a glassy carbon electrode, and a NO2-MIL-53(Cu-Al) covering the surface of the glassy carbon electrode.

[0007] The NO2-MIL-53(Cu-Al) comprises an organic carboxylic acid ligand and two coordination metals.

[0008] The organic carboxylic acid ligand is nitroterephthalic acid.

[0009] The two coordination metals are copper and aluminum.

[0010] The NO2-MIL-53(Cu-Al) has a hedgehog spherical structure, which is formed by clustering of needle leaf structures, and the size of the hedgehog ball is 13-17 μm.

[0011] The molar ratio of the two coordination metals copper and aluminum and the organic carboxylic acid ligand is 0.25-4:1:1.5.

[0012] The electrochemical sensing electrode is obtained by dissolving the prepared NO2-MIL-53(Cu-Al) in distilled water for ultrasonic treatment to obtain a dispersion liquid, dropping the NO2-MIL-53(Cu-Al) dispersion liquid on the surface of a glassy carbon electrode, and naturally drying at room temperature to obtain a NO2-MIL-53(Cu-Al) GCE electrochemical modified electrode.

[0013] The concentration of the NO2-MIL-53(Cu-Al) dispersion liquid is 1 g / L.

[0014] The NO2-MIL-53(Cu-Al) powder is prepared by mixing Cu(NO3)2·3H2O, Al(NO3)2·9H2O and nitroterephthalic acid in water to obtain a mixed solution, ultrasonic treatment, and then transferred into a high-temperature hydrothermal kettle for solvothermal reaction, washed with methanol and DMF after cooling, and finally oven dried to obtain NO2-MIL-53(Cu-Al) solid powder.

[0015] The ratio of the inorganic soluble salt of copper, the inorganic soluble salt of aluminum and nitroterephthalic acid is 1:1:1.5.

[0016] The reaction temperature of the reaction kettle is 120-200℃, and the heating time is 5-6 h.

[0017] The method for detecting NHDC by the electrode includes the following steps: taking the electrochemical modified electrode as a working electrode, assembling a three-electrode system with a saturated mercury-mercury electrode and a platinum electrode, taking a pH=5.8 Mcllvaine solution as an electrolyte, determining NHDC and SUC by a DPV method, recording the oxidation peak current value corresponding to different concentrations of the detection substance, and fitting to obtain a linear equation of the concentration of the detection substance and the oxidation current value i pa1 .

[0018] When the concentration of NHDC is in the range of 0.16-2.0 μmol / L, the fitting linear equation is i pa1 (10 -7 A) = 0.2114c NHDC (μmol / L) + 0.9565. When the concentration of NHDC is in the range of 2.0-30.0 μmol / L, the fitting linear equation is i pa1 (10 -7 A): 0.0189cNHDC (μmol / L) + 1.3462R 2 = 0.9921;

[0019] SUC concentration is 0.06-0.3mmol / L, the fitting linear equation i pa2 (10 -8 A) = 2.1535c suc (mmol / L) + 1.5057, SUC concentration is 0.35-7mmol / L, the fitting linear equation i pa2 (10 -8 A) = 2.2373c suc (mmol / L) + 1.6266, R 2 = 0.9914;

[0020] It is detected that under the condition of 3 times signal-to-noise ratio (S / N), the detection limit is calculated according to the formula , wherein σ represents the standard deviation of the peak current value of the lowest concentration under the running curve, and R represents the slope of the fitting curve.

[0021] The advantages and beneficial effects of the present application are as follows:

[0022] The bimetallic organic framework material can be obtained by one-step hydrothermal method, without using precise and expensive instruments, and the method is simple, low in cost, green and environmentally friendly. In addition, the catalytic ability and conductivity of the material are improved by the coupling effect between the two metal ions, the electrochemical response signal of the complex sweetener on the modified electrode surface is enhanced, and the effect is higher than that of a single metal component. And when the bimetallic organic framework material is applied to the detection of complex sweeteners, it can be observed that there is a good linear relationship between the oxidation current and the concentration of NHDC and SUC within a certain concentration range, and the detection limit is low. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 It is the preparation process of NO2-MIL-53(Cu-Al) GCE and the schematic diagram of detecting complex sweeteners (NHDC and SUC);

[0024] Figure 2X-ray diffraction (XRD) and Fourier transform infrared spectra (FT-IR) of (a) and (b) are NO2-MIL-53(Cu-Al), NO2-MIL-53(Cu), NO2-MIL-53(Al), respectively; (c) represents SEM images of NO2-MIL-53(Cu), (d) represents SEM images of NO2-MIL-53(Cu-Al) (Cu / Al = 4:1), (e) SEM images of NO2-MIL-53(Cu-Al) (Cu / Al = 2:1), (f) represents SEM images of NO2-MIL-53(Cu-Al) (Cu / Al = 1:1), (g) represents SEM images of NO2-MIL-53(Cu-Al) (Cu / Al = 1:2), (h) represents SEM images of NO2-MIL-53(Cu-Al) (Cu / Al = 1:4), (I) represents SEM images of NO2-MIL-53(Al);

[0025] Figure 3 N2adsorption-desorption isotherms of (a), (c), (e) are NO2-MIL-53(Cu), NO2-MIL-53(Al) and NO2-MIL-53(Cu-Al) (Cu / Al = 1:1), respectively, and (b), (d), (f) are pore size distribution of NO2-MIL-53(Cu), NO2-MIL-53(Al) and NO2-MIL-53(Cu-Al) (Cu / Al = 1:1), respectively;

[0026] Figure 4 XPS spectra of NO2-MIL-53(Cu-Al), (a) is the full spectrum, (b) is the C1s spectrum, (c) is the N1s spectrum, (d) is the O1s spectrum, (e) is the Cu 2p spectrum and (f) is the Al 2p spectrum;

[0027] Figure 5 CV response curves of NO2-MIL-53(Cu-Al) GCE (Cu / Al = 1:1), NO2-MIL-53(Cu) GCE and NO2-MIL-53(Al) GCE in pH = 5.8 MBS buffer solution containing 5 μmol / L NHDC and 1.0 mmol / L SUC;

[0028] Figure 6 Electrochemical impedance spectrograms of NO2-MIL-53(Cu-Al) GCE, NO2-MIL-53(Cu) GCE and NO2-MIL-53(Al) GCE and bare glassy carbon electrode in 5 mmol / L Fe[(CN)6] 3- / 4- and 0.1 mol / L KCl solution, and the inset is the fitting equivalent circuit diagram of the impedance curve;

[0029] Figure 7 Fig. 6 is a diagram of the anti-interference, reproducibility and stability of the NO2-MIL-53(Cu-Al) GCE, wherein (a) is a diagram of the anti-interference of the NO2-MIL-53(Cu-Al) GCE, (b) is a diagram of the reproducibility of the NO2-MIL-53(Cu-Al) GCE, and (c) is a diagram of the stability of the NO2-MIL-53(Cu-Al) GCE;

[0030] Figure 8 Fig. 7 is a diagram of the DPV curves of the NO2-MIL-53(Cu-Al) GCE for detecting 1.0 mmol / L SUC and different concentrations of NHDC in a pH=5.8 MBS buffer solution, wherein the detection concentrations of NHDC from bottom to top are 0.16 μmol / L, 0.24 μmol / L, 0.32 μmol / L, 0.40 μmol / L, 0.80 μmol / L, 2.0 μmol / L, 5.0 μmol / L, 8.0 μmol / L, 13.0 μmol / L, 20.0 μmol / L and 30.0 μmol / L; (b) is a diagram of the linear fitting relationship between the current and the concentration of NHDC in the range of 0.16-30 μmol / L; (c) is a diagram of the DPV curves of the NO2-MIL-53(Cu-Al) GCE for detecting 5.0 μmol / L NHDC and different concentrations of SUC in a pH=5.8 MBS buffer solution, wherein the detection concentrations of SUC from bottom to top are 0.06 mmol / L, 0.08 mmol / L, 0.1 mmol / L, 0.14 mmol / L, 0.16 mmol / L, 0.18 mmol / L, 0.20 mmol / L, 0.25 mmol / L, 0.30 mmol / L, 0.35 mmol / L, 0.40 mmol / L, 0.45 mmol / L, 1.0 mmol / L, 2.0 mmol / L, 4.0 mmol / L, 6.0 mmol / L and 7.0 mmol / L; (d) is a diagram of the linear fitting relationship between the current and the concentration of SUC in the range of 0.06-7.0 μmol / L. pa1 and c NHDC ; (d) is a diagram of the linear fitting relationship between the current and the concentration of SUC in the range of 0.06-7.0 μmol / L. pa2 and c SUC . DETAILED DESCRIPTION

[0031] The application will be further described in detail below in combination with the drawings and specific examples.

[0032] Example 1: Preparation of the NO2-MIL-53(Cu) modified electrode

[0033] 8 mmol of copper nitrate trihydrate (Cu(NO3)2·3H2O) and 6 mmol of nitroterephthalic acid (NO2-H2BDC) were dissolved in 30 mL of double distilled water, 10 mmol of urea was added after ultrasonic for 30 min, and the solution was dissolved by ultrasonic and then placed in a 100 mL reactor, heated at 150 ℃ for 6 h, and naturally cooled to room temperature. Each was washed with methanol and DMF for 3 times, dried at 60 ℃, and a blue solid powder was collected, which was NO2-MIL-53(Cu).

[0034] The glassy carbon electrode (GCE) was polished with 0.3 μm and 0.05 μm Al2O3 powder on the leather in turn, and then the glassy carbon electrode was ultrasonically treated in dilute nitric acid solution (volume ratio 1:1), ethanol aqueous solution and double distilled water for 1 min, and then naturally air-dried. First, 1 mg of NO2-MIL-53(Cu) material was dispersed in 1 mL of double distilled water, and a uniform dispersion was obtained after ultrasonic for 30 min. Then, 6 μL of the suspension was dropped on the surface of the cleaned glassy carbon electrode with a pipette, and the NO2-MIL-53(Cu) modified electrode was obtained after natural drying at room temperature. Then, it was used as a working electrode, and a saturated calomel electrode and a platinum electrode were assembled into a three-electrode system, and the electrolyte was a pH = 5.8 MBS solution prepared by mixing 0.1 mol / L citric acid and 0.2 mol / L disodium hydrogen phosphate solution according to a certain volume, and was connected to an electrochemical workstation for electrochemical detection of complex sweeteners (NHDC and SUC).

[0035] Example 2: Preparation of NO2-MIL-53(Al) modified electrode

[0036] 8 mmol of aluminum nitrate nonahydrate (Al(NO3)3·9H2O) and 6 mmol of nitroterephthalic acid (NO2-H2BDC) were dissolved in 30 mL of double distilled water, 10 mmol of urea was added after ultrasonic for 30 min, and the solution was dissolved by ultrasonic and then placed in a 100 mL reactor, heated at 150 ℃ for 6 h, and naturally cooled to room temperature. Each was washed with methanol and DMF for 3 times, dried at 60 ℃, and a blue solid powder was collected, which was NO2-MIL-53(Cu).

[0037] The glassy carbon electrode was polished with 0.3 μm and 0.05 μm Al2O3 powder on a leather, and then was ultrasonically cleaned in dilute nitric acid solution (volume ratio 1:1), ethanol aqueous solution and double distilled water for 1 min, respectively, and was naturally dried. First, 1 mg of NO2-MIL-53(Al) material was dispersed in 1 mL of double distilled water, and a uniform suspension was obtained after ultrasonic treatment for 30 min. Then, 6 μL of the suspension was dropped onto the surface of the cleaned glassy carbon electrode using a pipette, and the NO2-MIL-53(Al) modified electrode was obtained after natural drying at room temperature. Then, it was used as a working electrode, and a saturated calomel electrode and a platinum electrode were assembled into a three-electrode system. The electrolyte was a pH = 5.8 MBS solution prepared by mixing 0.1 mol / L citric acid and 0.2 mol / L sodium hydrogen phosphate solution according to a certain volume, and was connected to an electrochemical workstation for electrochemical detection of the complex sweetener (NHDC and SUC).

[0038] Example 3 Preparation of NO2-MIL-53(Cu-Al) modified electrode

[0039] (1) Preparation of NO2-MIL-53(Cu-Al) material

[0040] 4 mmol of copper nitrate trihydrate (Cu(NO3)2·3H2O), 4 mmol of aluminum nitrate nonahydrate (Al(NO3)3·9H2O) and 6 mmol of nitroterephthalic acid (NO2-H2BDC) were dissolved in 30 mL of double distilled water, and after ultrasonic treatment for 30 min, 10 mmol of urea was added, and after ultrasonic dissolution, it was placed in a 100 mL reaction kettle and heated at 150°C for 6 h, and then naturally cooled to room temperature. Methanol and DMF were each washed 3 times, and dried at 60°C. Blue solid powder was collected, which was NO2-MIL-53(Cu-Al) (Cu / Al = 1:1). Unless otherwise specified, Cu / Al = 1:1 by default.

[0041] (2) Preparation of NO2-MIL-53(Cu-Al) modified electrode

[0042] The glassy carbon electrode (GCE) was polished with 0.3 μm and 0.05 μm Al2O3 powder on a leather, and then was ultrasonically cleaned in dilute nitric acid solution (volume ratio 1:1), ethanol aqueous solution and double distilled water for 1 min, respectively, and was naturally dried. First, 1 mg of NO2-MIL-53(Cu-Al) material was dispersed in 1 mL of double distilled water, and a uniform dispersion was obtained after ultrasonic treatment for 30 min. Then, 6 μL of the suspension was dropped onto the surface of the cleaned glassy carbon electrode using a pipette, and the NO2-MIL-53(Cu-Al) modified electrode was obtained after natural drying at room temperature. (The concentration of the dispersion was 1 mg / mL, and the volume was 6 μL.) Then, the electrode was used as a working electrode, and was assembled into a three-electrode system with a saturated calomel electrode and a platinum electrode. The electrolyte was a pH = 5.8 MBS solution prepared by mixing 0.1 mol / L citric acid and 0.2 mol / L disodium hydrogen phosphate solution according to a certain volume, and was connected to an electrochemical workstation for the electrochemical detection of the compound sweetener (NHDC and SUC).

[0043] Examples 4-7

[0044] The NO2-MIL-53(Cu-Al) modified electrode was prepared according to the method of Example 3, except that different molar ratios of copper nitrate trihydrate and aluminum nitrate nonahydrate (Cu / Al = 1:4, 1:2, 2:1, 4:1) were weighed according to the same method under the condition that the total amount of copper and aluminum ions was constant, and NO2-MIL-53(Cu-Al) with different copper-aluminum ratios was synthesized.

[0045] The different modified electrodes prepared in Examples 1, 2, 3 and 4-7 were characterized by XRD, FT-IR, SEM, BET and XPS, and the electrochemical behavior of the compound sweetener (NHDC and SUC) was studied, the electrochemical characterization was performed, and the anti-interference, reproducibility and stability performance tests were carried out.

[0046] Test I: XRD and FT-IR characterization

[0047] As Figure 2As shown in (a) of FIG. 1, using XRD to analyze the crystal structure of NO2-MIL-53(Cu-Al) (Example 3), NO2-MIL-53(Cu) and NO2-MIL-53(Al), it can be seen that the peaks corresponding to NO2-MIL-53(Cu) and NO2-MIL-53(Al) materials appear in the pattern of NO2-MIL-53(Cu-Al), wherein 2θ = 8.3°, 8.6°, 12.6°, 13.6°, 14.9°, 16.7° and 17.1° correspond to (001), (110), (011), (01-1), (101), (101) and (220) crystal faces, respectively, which indicates that the bimetallic material NO2-MIL-53(Cu-Al) is successfully synthesized, and no obvious diffraction peaks of residues or impurities are observed in the XRD pattern, indicating that the synthesized substance has high purity.

[0048] As shown in (a) of FIG. 1, using XRD to analyze the crystal structure of NO2-MIL-53(Cu-Al) (Example 3), NO2-MIL-53(Cu) and NO2-MIL-53(Al), it can be seen that the peaks corresponding to NO2-MIL-53(Cu) and NO2-MIL-53(Al) materials appear in the pattern of NO2-MIL-53(Cu-Al), wherein 2θ = 8.3°, 8.6°, 12.6°, 13.6°, 14.9°, 16.7° and 17.1° correspond to (001), (110), (011), (01-1), (101), (101) and (220) crystal faces, respectively, which indicates that the bimetallic material NO2-MIL-53(Cu-Al) is successfully synthesized, and no obvious diffraction peaks of residues or impurities are observed in the XRD pattern, indicating that the synthesized substance has high purity. Figure 2 As shown in (b) of FIG. 1, the FT-IR spectra of NO2-MIL-53(Cu-Al) (Example 3), NO2-MIL-53(Cu), NO2-MIL-53(Al) and nitroterephthalic acid (NO2-H2BDC) are obviously different. The hydroxyl group of the carboxyl group in NO2-H2BDC can be observed in the broadband of 2500-3300 cm -1 , however, this phenomenon disappears in NO2-MIL-53(Cu-Al) (Example 3), NO2-MIL-53(Cu) and NO2-MIL-53(Al), reflecting the deprotonation process of NO2-H2BDC in the process of MOFs formation, wherein the band at 1596.6 cm -1 is the symmetric stretching vibration from COO - , indicating successful coordination between the ligand and the metal ion. In addition, the same band as NO2-H2BDC appears near 1396.5 cm -1 in NO2-MIL-53(Cu-Al) (Example 3), NO2-MIL-53(Cu) and NO2-MIL-53(Al) materials, which can be attributed to the anti-symmetric stretching vibration of NO2. Overall, the FT-IR spectrum also confirms the successful synthesis of NO2-MIL-53(Cu-Al) (Example 3), NO2-MIL-53(Cu) and NO2-MIL-53(Al).

[0049] Test two: SEM characterization

[0050] As shown in (a) of FIG. 1, using XRD to analyze the crystal structure of NO2-MIL-53(Cu-Al) (Example 3), NO2-MIL-53(Cu) and NO2-MIL-53(Al), it can be seen that the peaks corresponding to NO2-MIL-53(Cu) and NO2-MIL-53(Al) materials appear in the pattern of NO2-MIL-53(Cu-Al), wherein 2θ = 8.3°, 8.6°, 12.6°, 13.6°, 14.9°, 16.7° and 17.1° correspond to (001), (110), (011), (01-1), (101), (101) and (220) crystal faces, respectively, which indicates that the bimetallic material NO2-MIL-53(Cu-Al) is successfully synthesized, and no obvious diffraction peaks of residues or impurities are observed in the XRD pattern, indicating that the synthesized substance has high purity. Figure 2(cI) shows the SEM characterization results of NO2-MIL-53(Cu), NO2-MIL-53(Al), and NO2-MIL-53(Cu-Al) with different Cu / Al molar ratios on a glassy carbon substrate. Figure 2 In the SEM image (c), the NO2-MIL-53(Cu) material on the glassy carbon substrate surface can be observed to form a flower-like structure composed of blocky stacks, with a particle size of approximately 7.5 μm; from Figure 2 In the SEM image of NO2-MIL-53(Cu-Al) (Example 3), it can be clearly seen that its microstructure has become a hedgehog ball composed of needle-like clusters, with the size of the ball being approximately in the range of 13-17 μm. Figure 2 Image (I) is a SEM image of NO2-MIL-53(Al), which is a multilayered spherical structure with numerous layers, approximately 20 μm in size. Other SEM images with different Cu / Al molar ratios were observed... Figure 2 As shown in (d), (e), (g), and (h), when the molar ratio of Cu decreases while the molar ratio of Al increases, the basic shape of the sample remains unchanged, but it gradually changes from a blocky stack to a needle-like stack and finally to a spherical structure formed by a layered stack. Furthermore, by comparing the morphology and structure of the synthesized materials, it can be seen that the NO2-MIL-53(Cu-Al) (Example 3) material has a relatively rough surface, which can provide more active sites, increase the specific surface area of ​​the bare glassy carbon electrode, and facilitate the adsorption of active materials, thereby enhancing the oxidation current signal response of NHDC and SUC.

[0051] Test 3: BET Characterization

[0052] The specific surface area and pore structure of NO2-MIL-53(Cu), NO2-MIL-53(Al), and NO2-MIL-53(Cu-Al) (Example 3) materials were evaluated using N2 adsorption-desorption isotherms. Figure 3 As shown in (a), the N2 adsorption-desorption isotherm of NO2-MIL-53(Cu) exhibits a large area of ​​negative adsorption values ​​for nitrogen. This phenomenon may be due to the sample having no adsorption or a very small adsorption amount, in which case the adsorption value should be near 0. Combined with instrumental errors, this can lead to negative values ​​indicating adsorption points. Therefore, the adsorption of this type of sample is almost negligible, suggesting a very small specific surface area and indicating a very low number of micropores and mesopores. Testing shows that its specific surface area is 1.55 m². 2 / g. From Figure 3 As shown in (b), the pore size distribution of this material is above 50 nm. Furthermore, through... Figure 3As shown in (c) and (e), the NO2-MIL-53(Al) and NO2-MIL-53(Cu-Al) (Example 3) materials exhibit typical type IV isotherms and type H3 hysteresis loops, indicating that both prepared samples possess mesoporous structures. Tests revealed their specific surface areas to be 111.95 m² / s². 2 / g and 121.56m 2 / g, by Figure 3 From (d) and (f), we can see that the pore size distributions of NO2-MIL-53(Al) and NO2-MIL-53(Cu-Al) materials are concentrated in the ranges of 0-40 nm and 0-50 nm, respectively. BET characterization data show that NO2-MIL-53(Cu-Al) (Example 3) has a large specific surface area, which can increase the effective contact area between the electrode and the analyte, thereby improving the final detection performance of the electrode.

[0053] Test 4: XPS Characterization

[0054] To further investigate the room-temperature surface elemental composition and chemical valence state of NO2-MIL-53 (Cu-Al) material, XPS measurements were performed, such as... Figure 4 As shown, where, Figure 4 Image (a) shows the full spectrum of NO2-MIL-53 (Cu-Al) material, proving the presence of C, N, O, Cu, and Al elements in the sample. Figure 4 (b) shows the high-resolution XPS spectrum of C1s, with three main peaks located at 283.36, 284.80, and 287.27 eV, which are attributed to C=C, C=C, and CO bonds, respectively. Figure 4 (c) provides the high-resolution XPS spectrum of N1s, where the peak at 399.02 eV is attributed to the NO bond, and the peak at 404.43 eV is attributed to the CN bond. Figure 4 (d) in the image represents the high-resolution XPS spectrum of O1s. The peaks at binding energies of 530.08 and 531.90 eV correspond to the MO (where M is a metal, Cu or Al) and C=O bonds, respectively. Figure 4 The high-resolution XPS spectrum of Cu 2p (e) shows that the peaks at 931.60 and 951.17 eV can be attributed to the binding energies of Cu 2p3 / 2 and Cu 2p1 / 2, respectively. Fitting the Cu 2p spectrum led to the identification of the two components. Further peak fitting of the orbital peaks of Cu 2p3 / 2 and Cu 2p1 / 2 confirmed that Cu... 0 and Cu 2+ The presence of ions. Among them, Cu... 0 The binding energies of the matter are located at 933.40 and 953.17 eV, while Cu 2+The binding energies of the matter are located at 938.33, 942.04, and 960.96 eV. Figure 4 (f) in the figure is the high-resolution XPS spectrum of Al 2p. From the graph, it can be observed that the peak at 72.99 eV can be attributed to Al. 3+ The peak at 76.25 eV can be attributed to Al. 0 This test result largely corresponds to the results of XRD and FT-IR tests.

[0055] Test 5: Electrochemical Behavior Study of Compound Sweeteners (NHDC and SUC)

[0056] The CV responses of the compound sweeteners (NHDC and SUC) on NO2-MIL-53(Cu-Al)GCE (Example 3) (Blank) [representing blank buffer solution], NO2-MIL-53(Cu-Al)GCE (Example 3), NO2-MIL-53(Cu)GCE and NO2-MIL-53(Al)GCE are as follows: Figure 5 As shown, the potential window is 0–1.0 V, the scan rate is 0.05 V / s, and the scan is 2 revolutions. Figure 5 It can be seen that NO2-MIL-53(Cu-Al)GCE (Example 3) did not show any redox peaks in the blank buffer solution, indicating that the modified electrode material itself did not undergo a redox reaction. Furthermore, the glassy carbon electrodes modified with the single-metal materials NO2-MIL-53(Cu) and NO2-MIL-53(Al) exhibited poor electrochemical responses to NHDC and SUC within the potential window of 0.0-1.0 V. In contrast, the glassy carbon electrode modified with NO2-MIL-53(Cu-Al) (Example 3) showed four distinct redox peaks in two cycles of cyclic voltammetry testing. This may be because the synergistic effect of the bimetallic compounds can improve the catalytic activity and conductivity of the material, enhancing the electrochemical response signal of the compound sweetener on the modified electrode surface. In the first cycle, a strong irreversible oxidation peak (P0) was observed at 0.505 V. a1 Another irreversible strong oxidation peak (P) was obtained at 0.838 V. a2 A reduction peak (P) was obtained at 0.143V. c In the second consecutive cycle, a new reduction peak (P0) appeared at 0.185V, similar to the one in the first cycle. c The oxidation peak corresponding to ) (P a3 In other words, P a2 and P c These are a pair of reversible redox peaks. The graph shows that P in the second cycle... a1 and P a2The current decreased compared to the first cycle, likely because the oxidation products of NHDC and SUC in the first cycle were irreversible, partially clogging the pores. Therefore, to better observe the test results, in subsequent experiments, the P in the first cycle was... a1 and P a2 It was used for the effect analysis of NHDC and SUC. In addition, in NO2-MIL-53 (Cu-Al) in Examples 4 to 7, there were also 4 redox peaks when Cu / Al = 1:4, 1:2, 2:1 and 4:1, but the peak current values ​​were very small and not as obvious as when Cu / Al = 1:1 in Example 3.

[0057] Detection 6: Electrochemical Characterization

[0058] Electrochemical impedance spectroscopy (EIS) is a powerful tool for studying the interfacial properties of electrodes with different modifications. Characteristic impedance semicircles controlled by charge transfer processes appear in the high-frequency region, while characteristic straight lines controlled by diffusion appear in the low-frequency region. Typically, the diameter of the semicircle reflects the charge transfer resistance (Rct). Figure 6 As shown, bare glassy carbon electrode (Bare GCE), NO2-MIL-53(Cu-Al)GCE (Example 3), NO2-MIL-53(Cu)GCE, and NO2-MIL-53(Al)GCE were tested in a solution containing 5 mmol / L Fe[(CN)6]. 3- / 4- Electrochemical impedance spectroscopy (EIS) was performed in a 0.1 mol / L KCl solution, yielding Nyquist curves. After fitting with the equivalent circuit diagram, the charge transfer resistance (Rct) of the Bare GCE-modified electrode was found to be 295.0 Ω, while the Rct of NO2-MIL-53(Cu-Al)GCE (Example 3), NO2-MIL-53(Cu)GCE, and NO2-MIL-53(Al)GCE decreased to 7.6 Ω, 48.9 Ω, and 15.88 Ω, respectively. These EIS results demonstrate that both single-metal and bimetallic materials can enhance the conductivity of bare glassy carbon electrodes, but NO2-MIL-53(Cu-Al)GCE (Example 3) exhibits a stronger enhancing effect on conductivity, a result consistent with CV data.

[0059] Test 7: Anti-interference test

[0060] The anti-interference ability of modified electrodes is an important indicator for evaluating their performance. For example... Figure 7 As shown, the DPV method was used to test the anti-interference ability of NO2-MIL-53(Cu-Al)GCE (Example 3) to simultaneously detect NHDC and SUC in various inorganic and organic substances. Figure 7 (a) shows the addition of 100 times the concentration of Na+ to an MBS buffer solution containing 10 μmol / L NHDC and 10 μmol / L SUC.+ , K + , Cl - , NO3 - , SO4 2- , Br - , CO3 2 , and 10 times CH3COO - , sucrose, glucose, DL-serine, L-serine and p-nitrophenol organic interferents, the peak value of the oxidation current intensity of NO2-MIL-53(Cu-Al) GCE (Example 3) after adding the interferents was measured, respectively. As can be seen from the figure, after adding 100 times the concentration of inorganic interferents and 10 times the concentration of organic matter, the i-t current curve did not increase or decrease significantly, and basically remained stable, indicating that NO2-MIL-53(Cu-Al) (Example 3) has high selectivity for accurate determination of NHDC and SUC.

[0061] Detection eight: reproducibility and stability detection

[0062] As shown in (b) of Figure 7 , the reproducibility of the modified electrode was evaluated by DPV detection of MBS buffer solution containing 5 μmol / L NHDC and 1.0 mmol / L SUC using the same preparation process with seven independently prepared parallel modified electrodes, and the relative standard deviations (RSD) of i pa1 and i pa2 were 4.16% and 4.65%, respectively. Secondly, in order to further investigate the reproducibility of the modified electrode, the same modified electrode was used for continuous DPV test for 7 times under the optimal experimental conditions, and the relative standard deviations (RSD) of i pa1 and i pa2 were 3.32% and 4.30%, respectively. In addition, the NO2-MIL-53(Cu-Al) modified electrode was placed at room temperature for one week, and the change of DPV signal was recorded every day to observe the stability of the modified electrode. As can be observed from (c) of Figure 7 , the current value of p a1 still can retain 84.2% of the initial value, and the current value of p a2 still can retain 89.6% of the initial value. The above results show that the NO2-MIL-53(Cu-Al) GCE prepared in this experiment has good reproducibility and stability in detecting complex sweeteners.

[0063] Effect example: working curve and NHDC detection in actual sample

[0064] As shown in Figure 8As shown, based on the NO2-MIL-53(Cu-Al) modified electrode, differential pulse voltammetry (DPV) was used to determine NHDC and SUC, and the peak current and peak potential corresponding to the test substance were recorded. Under the condition of 3 times of signal-to-noise ratio (S / N), the detection limit was calculated according to the formula , wherein σ represents the standard deviation of the peak current value of the lowest concentration under the running curve.

[0065] Based on the DPV detection curve of the sensor constructed based on NO2-MIL-53(Cu-Al) for SUC / NHDC, it can be seen that NHDC and SUC have small cross-reactivity in the mixed solution. Within a suitable concentration range, NO2-MIL-53(Cu-Al) can selectively determine SUC or NHDC without mutual interference. Therefore, by DPV, it can be observed that when the concentration of SUC is fixed and the concentration of NHDC is changed, although cross-reaction may occur, the standard solution of NHDC still has a good linear relationship within the concentration range of 0.16-30.0 μmol / L, and thus the detection of the amount of NHDC within the concentration range of 0.16-30.0 μmol / L is not disturbed by the concentration of SUC. Figure 8 (a) in FIG. 6 is a DPV curve of the determination of the standard solution containing 1.0 mmol / L SUC and the standard solution of NHDC with a concentration in the range of 0.16-30.0 μmol / L. As can be seen from the figure, the oxidation peak current of NHDC increases with the increase of its concentration. Figure 8 (b) in FIG. 6 gives the linear relationship between the oxidation peak current (i pa1 ) of NHDC and its concentration (c NHDC ). Within the low concentration range of 0.16-2.0 μmol / L, the fitting linear equation is i pa1 (10 -7 A) = 0.2114c NHDC (μmol / L) + 0.9565, R 2 = 0.9945, R represents the slope of the fitting curve, and the detection limit is calculated to be 5.37 nmol / L; similarly, within the high concentration range of 2.0-30.0 μmol / L, the fitting linear equation is i pa1 (10 - 7 A): 0.0189c NHDC (μmol / L) + 1.3462R 2 = 0.9921.

[0066] Similarly, in the appropriate concentration range, NO2-MIL-53(Cu-Al) can selectively determine SUC or NHDC without interference with each other. By DPV, it can be observed that when the concentration of NHDC is fixed and the concentration of SUC is changed, although there is a cross-reaction phenomenon, the standard solution of SUC still has a good linear relationship in the concentration range of 0.06-7.0 mmol / L, and the amount of SUC detected in this concentration range is not disturbed by the concentration of NHDC. Figure 8 (c) in FIG. 6 is the DPV curve of the determination containing 5.0 μmol / L NHDC standard solution and SUC standard solution with the concentration in the range of 0.06-7.0 mmol / L. It can be seen from the figure that the oxidation peak current of SUC increases with the increase of its concentration. Figure 8 (d) in FIG. 6 gives the linear relationship between the oxidation peak current (i pa2 ) of SUC and its concentration (c SUC ) in the range of 0.06-7.0 mmol / L. In the low concentration range of 0.06-0.3 mmol / L, the fitting linear equation is i pa2 (10 -8 A) = 2.1535c suc (mmol / L) + 1.5057, R 2 = 0.9929, and the detection limit is 2.38 μmol / L; in the high concentration range of 0.35-7.0 mmol / L, the fitting linear equation is i pa2 (10 -8 A) = 2.2373c suc (mmol / L) + 1.6266, R 2 = 0.9914.

[0067] In order to investigate the application potential of NO2-MIL-53(Cu-Al) GCE, it is applied to the detection of the reconstituted sweetener (NHDC and SUC) in the drug samples obtained from the local drugstore. Under the optimized experimental conditions, the DPV method is used to test in the MBS solution containing the reconstituted sweetener for many times, and the results show that the reconstituted sweetener (NHDC and SUC) is detected in the drug samples. In addition, the recovery rate of the method and the content of NHDC and SUC in the drug samples are obtained by the standard addition recovery method. The sample standard addition recovery refers to taking two parts in the same sample, one of which adds a quantitative standard substance of the measured component; both are analyzed according to the same analysis steps, and the difference between the results of the sample with standard addition and the sample without standard addition is the sample standard addition recovery rate.

[0068] The standard addition recovery rate = (the measured value of the sample with standard addition - the measured value of the sample) ÷ the standard addition amount × 100%.

[0069] As shown in Table 1, the i of NHDC in the sample was measured by the DPV method in electrochemistry. pa1 Then substitute the value into the linear equation: i pa1 (10 -7 A) = 0.2114c NHDC (μmol / L) + 0.9565, R 2 =0.9945 to obtain the detection value; when using the spiked recovery method to detect NHDC in the drug sample, if the NHDC concentration of the sum of the detection value and the amount added is less than 2.0 μmol / L, i is added. pa1 (10 -7 A) = 0.2114c NHDC (μmol / L) + 0.9565, R 2 =0.9945. The measured value is obtained from the equation. When the NHDC concentration of the sum of the detected value and the added amount is greater than 2.0 μmol / L, i is added. pa1 (10 -7 A): 0.0189c NHDC (μmol / L)+1.3462R 2 The measured value was obtained from the equation = 0.9921. The results showed that the NHDC content in the drug sample was 0.631 μg / mg, equivalent to 1.03 μM (the average of NHDC in three samples), with an average recovery rate in the range of 98.8%-103.2%. Simultaneously, the NHDC content in the same batch of drug samples was determined by high-performance liquid chromatography (HPLC) to be 0.678 μg / mg, with an error of 6.93%. As shown in Table 2, the i-value of SUC in the sample was determined by the DPV method in electrochemistry. pa2 Then substitute the value into the linear equation: i pa2 (10 - 8 A) = 2.1535c suc (mmol / L) +1.5057, R 2 =0.9929, the detection value was obtained; when using the spiked recovery method to detect SUC in the drug sample, when the SUC concentration of the sum of the detection value and the amount added is greater than 0.35 mmol / L, i is added. pa2 (10 -8 A) = 2.2373c suc (mmol / L) +1.6266, R 2= 0.9914 The measured value obtained from the equation. The results showed that the content of SUC in the drug sample was 2.09 pg / mg, which was equivalent to 0.00525 mM (the average value of SUC in 3 samples) of SUC, and the average recovery rate was in the range of 100.3%-101.8%. At the same time, the content of SUC in the same batch of drug sample was 2.23 pg / mg by high performance liquid chromatography, with an error of 6.28%. These results showed that the NO2-MIL-53(Cu-Al) modified electrode was very suitable for the detection of complex sweeteners (NHDC and SUC) in actual samples.

[0070] Table 1 Detection of NHDC in drug samples by standard addition recovery method (n = 3)

[0071]

[0072] Table 2 Detection of SUC in drug samples by standard addition recovery method (n = 3)

[0073]

Claims

1. A method for detecting a complex sweetener, characterized by, The pH = 5.8 Mcllvaine solution is used as an electrolyte, and the DPV method is used to measure NHDC and SUC and products containing NHDC and SUC, and the electrochemical sensing electrode includes a glassy carbon electrode and a NO2-MIL-53(Cu-Al) covered on the surface of the glassy carbon electrode; The NO2-MIL-53(Cu-Al) includes an organic carboxylic acid ligand and two coordination metals; The organic carboxylic acid ligand is nitroterephthalic acid; The two coordination metals are copper and aluminum; The NO2-MIL-53(Cu-Al) has a hedgehog spherical structure, and the hedgehog spherical structure is formed by clustering of needle leaf structures, and the size of the hedgehog ball is 13-17 μm; The NO2-MIL-53(Cu-Al) is prepared by mixing an inorganic soluble salt of copper, an inorganic soluble salt of aluminum and nitroterephthalic acid in water, adding urea to prepare a mixed solution, ultrasonic treatment, then transferring into a reaction kettle for hydrothermal reaction, washing and drying after cooling to obtain a NO2-MIL-53(Cu-Al) solid powder, and the reaction kettle reaction temperature is 120℃-200℃.

2. The method of claim 1, wherein the complex sweetener is a combination of sucralose and acesulfame potassium. The molar ratio of the two coordination metals copper and aluminum and the organic carboxylic acid ligand is 0.25-4:1:1.

5.

3. The method of claim 1, wherein the detecting is performed by a method selected from the group consisting of mass spectrometry, nuclear magnetic resonance, and chromatography. The electrochemical sensing electrode is prepared by dissolving the prepared NO2-MIL-53(Cu-Al) in distilled water and ultrasonic treatment to obtain a dispersion, dropping the NO2-MIL-53(Cu-Al) dispersion on the surface of a glassy carbon electrode, and naturally drying at room temperature to obtain a NO2-MIL-53(Cu-Al) GCE electrochemical modified electrode.

4. The detection method according to claim 3, characterized in that, The concentration of the NO2-MIL-53(Cu-Al) dispersion is 1 g / L.

5. The detection method according to claim 4, characterized in that, The ratio of the inorganic soluble salt of copper, the inorganic soluble salt of aluminum and nitroterephthalic acid is 1:1:1.

5.

6. The detection method according to claim 5, characterized in that, The heating time of the hydrothermal reaction is 5h-6h.

7. The method of detecting according to claim 3, comprising the steps of: The electrochemical modified electrode is used as a working electrode, and a saturated calomel electrode and a platinum electrode are assembled into a three-electrode system. A pH = 5.8 Mcllvaine solution is used as an electrolyte, and NHDC and SUC are determined by using a DPV method. Oxidation peak current values corresponding to different concentrations of the detection substance are recorded, and a linear equation of the concentration of the detection substance and the oxidation current value i pa1 is fitted. NHDC concentration in the range of 0.16-2.0 μmol / L, the fitting linear equation is: ipa1 = 0.2114c NHDC + 0.9565, NHDC concentration in the range of 2.0-30.0 μmol / L, the fitting linear equation is: ipa1 =0.0189c NHDC + 1.3462, R 2 =0.9921; CNHDC is in the unit of μmol / L, ipa1 is in the unit of 10 -7 A; The fitting linear equation when the SUC concentration is in the range of 0.06-0.3 mmol / L is ipa2 = 2.1535 c suc + 1.5057, and the fitting linear equation when the SUC concentration is in the range of 0.35-7 mmol / L is: ipa2 = 2.2373 c suc + 1.6266, R 2 = 0.9914. The unit of SUC is mmol / L; the detection limit is calculated according to the formula The unit of SUC is mmol / L; the detection limit is calculated according to the formula σ The standard deviation of the peak current value of the lowest concentration under the running curve, R The slope of the fitted curve, the unit of ipa2 is 10 -8 A.

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